A large number of coffee machines with an integrated coffee grinding device or which have said device as a supplementary built-on unit as well as an essentially automatic hot-water feed to the brewing device are known. One of these coffee machines is for example described in DE 3615158C1. With the known coffee machine coffee powder is fed through the actuation of a grinder into a brewing chamber, the brewing chamber is closed by a displaceable piston until a specified contact pressure of the piston on the coffee powder cake is reached and then the brewing water is passed through. If it is found after repeated preparation cycles that too little or too much coffee powder has been fed into the brewing chamber, the grinding time of the grinder is automatically shortened or lengthened under the action of a programmable control device. Simultaneously or in addition to this, a flow measuring element measures the amount of the fed brewing water and the control device measures the time required for it. If this time period deviates in a positive or negative sense from a specified set time period, a readjustment is made after a few preparation cycles in that the grinding grade carried out by the grinder is automatically set to “finer” or “coarser”. Smaller deviations within normal tolerances are not considered here in both cases.
The known coffee machine is formed as a fully automatic machine which is intended to simplify the user's activities associated with the preparation of various types of coffee. In principle the customer only needs to actuate a certain program key to obtain a desired coffee beverage. The control of the machine occurs according to factory-set programmed parameters which are designed for an optimum result for each certain type of beverage. These parameters are determined in advance through tasting and operational tests. These parameters may for example comprise the amount of fed coffee grounds, the grinding grade of the grounds, the contact pressure of the piston for piston-type brewers, the amount of water and the dispensing time. The maintenance of these parameters for each brewing process is monitored automatically and, with larger deviations, automatically corrected, without the user having to intervene.
Thus, it is also intended that the grinding grade of the grinder is modified automatically when the preparation time of a beverage deviates from an associated set value.
The grinding grade of the grounds brewed makes a decisive contribution to the coffee quality. If the grinding grade is too fine, then this can lead to the brewing process taking an unreasonable length of time or it even cancels, because the brewer is blocked. If it is too coarse, the brewing water passes through the grounds too quickly, producing a negative effect on the taste of the coffee. In particular, for the preparation of a good espresso a minimum contact time of the brewing water with the coffee is required. Also finely ground grounds offer more contact area with the brewing water, leading to better extraction and more aroma. In addition, for the development of the taste and the formation of a desirable creamy layer on the espresso it is necessary that the pressed grounds layer offers adequate resistance to the pump in order to establish a brewing pressure of between 6 and 9 bar. With a fine grinding grade this has a decisive favorable effect.
A factory setting to an “optimum” grinding grade is also not practicable, because with a defined setting for the grinding grade the grinding result can turn out to be very different due to the varying consistency of the sorts of beans used. Even with one and the same sort of bean the fineness of the ground coffee can significantly differ due to different degrees of roasting.
An automatic controlling device, as with the known coffee machine, is however, for example for household machines, too expensive, because a dedicated drive is required to adjust the grinding grade. Additionally, complex algorithms need to be saved in the controller which prevent a closed-loop control behavior that is too sensitive. Also the method for the measurement of the preparation time is not always suitable, because the different amounts prepared of different coffee beverages are not taken into account or a dedicated set value must be saved for each prepared amount.